Organic compounds trapped in aqueous fluid inclusions

Fluid inclusion samples from several Australian oil wells have been analysed to document the prevalence and composition of volatile hydrocarbons contained within aqueous inclusions. These results clearly establish that trapped palaeo formation waters can be a source of such compounds, which are frequently predominant in samples with a low content of oil-bearing inclusions. The apparent “anomalous” hydrocarbon distributions derived from aqueous inclusions contain abundant water-soluble compounds, such as benzene and toluene, which may originate from interaction of formation waters with subsurface petroleum accumulations. Aqueous inclusions are also often enriched in alkenes and oxygenated species, such as furan, which are minor constituents of petroleum but could form via secondary processes such as anoxic microbial degradation in formation waters. The co-occurrence of aqueous-derived organic compounds within samples containing oil inclusions suggest the need for caution when interpreting volatile hydrocarbon distributions. However, the presence of these components in samples from dry wells could be used as a tool to substantiate the proximity of a petroleum accumulation in an area which would otherwise be considered to have low prospectivity.